Method and article of freeze-drying



United States Patent METHOD AND ARTICLE OF FREEZE-DRYIN G Wilhelm Nerge, Gartenstr. 42, Rodenkirchen, Germany,

and Ulrich Hackenberg, Alter Trassweg 9, Bensberg- Refrath, Germany No Drawing. Filed Jan. 14, 1963, Ser. No. 251,055 Claims priority, application Germany, Jan. 13, 1962,

7 Claims. (Cl. 252-301.1)

This invention relates to a method and article of freezedrying, and more particularly to a method of freezedrying which prevents undesirable evaporation during the freeze-drying process.

Freeze-drying is a well known method of drying in which a material to be dried is first frozen and then placed inside an evacuated chamber. A water vapor removal apparatus such as, for example, a deep cooled ice condenser then adsorbs the water vapor produced by sublimation of the frozen water in the material to be dried. A pre-determined low pressure is continually maintained within the vacuum chamber during the drying process by means of conventional vacuum pumps communicating with the chamber.

Freeze-drying has the distinct advantage as a drying process of removing substances other than the sublimat ing frozen water in only extremely small quantities. However, in many freeze-drying applications the removal of even extremely small quantities of certain other substances may render the freeze-drying process inapplicable or at least seriously diminish its effectiveness.

It is, therefore, the object of this invention to'provide a freeze-drying method which greatly reduces the quantity of non-aqueous substances removed from the drying material either before or after the drying process.

One feature of this invention is the freeze-drying of aw'atery raidoactive waste material packaged in a material which is porous to the sublimating water vapor but impermeable to solid radioactive material.

Another feature of this invention is the freeze-drying of a food substance surrounded by another edible material which serves ta-s a filter for aromas of the core substance while allowing the removal therefrom of the sublimating frozen water during the freeze-drying process.

Another feature of this invention is a method of freezedrying wherein the material to be freeze-dried is coated with a porous substance having a sinter capacity and upon completion of the freeze-drying process the porous coating, while remaining under vacuum, is made impermeable by an application of heat which causes a closing of its pores.

Another feature of this invention is a method of freezedrying in which the material to be freeze-dried is provided during the drying process with a porous covering, and wherein the pores are closed by impregnation upon the completion of the freeze-drying process while the material remains under vacuum.

Still another feature of this invention is a freeze-drying process wherein the material being freeze-dried is covered during the process with a porous coating which includes a tempenable casting resin and upon completion of the freeze-drying process the porous coating is impregnated with a second resin hardening component.

These and other important objects and features of the present invention will become apparent upon a perusal of the following specification.

The present invention provides a method for safely and eificiently disposing of watery radioactive waste material. By drying radioactive waste according to conventional freeze-drying procedures, a good separation of the radioactive solid substance and the non-radioactive water content is achieved thereby greatly reducing the weight and volume of solid radioactive material to be disposed of. However, there still exists a significant contamination danger to the removed water vapor, the freeze-drying installation, installations which follow the freeze-drying apparatus in the decontamination process and so forth. Therefore, the present invention also provides a vapor porous filter covering for the radioactive waste material during the freeze-drying process. This covering serves to obstruct solid radioactive material while allowing the passage of sublima ting water vapors during the freezedrying process. Upon the completion of the freeze-drying process the filter covering serves the additional purpose of providing a package for the dried radioactive solid material so as to promote ease of handling as well as to prevent disturbance and undesirable migration of the dry and sometimes powdery radioactive material.

As an example only, chemical filtration paper can be used as a covering material for the radioactive waste. The paper will effectively limit potentially harmful movement of solid material while allowing the removal of sublim-ating water vapors necessary to the freeze-drying process.

This invention can also be practiced with respect to the freeze-drying of food materials. In this case, the food substance to be dried is covered with another edible material which then acts as an aroma filter for the core food substance during the drying operation. For example only, strawberries which are to be subjected to freeze-drying may be surrounded as uniformly as possible with a layer of strained applesauce. After having been frozen the combination is subjected to a freeze-drying operation initially resulting in the drying of the applesauce coating. This d-ry applesauce layer will then possess a large number of capillaries and surfaces which will function as a depth filter with a pronounced absorptive effect for the aromas of the drying strawberry core. In this way a much larger percentage of the strawberry aroma is maintained within the combined product and will be available as flavor upon a reconstitution thereof for later use.

Another successful example of this application is the use of an inexpensive potato filter covering for other more expensive vegetable food cores.

This method of freeze-drying foods has an additional advantage in that the coating material may serve to maintain the combined product in a given form after the drying process for ease of handling and storage. This is extremely important in the drying of food materials which are not stable enough to maintain their shape upon completion of the drying process. Further, in many cases the coating material may also prevent agglutination between individual pieces of the core food substance as well .as between the core food and the drying containers. In these applications it is preferable to use a covering of a diluted watery substance rather than one composed of a pap like substance.

The use of a covering for freeze-drying foods according to the present invention can also permit an increase in the maximum allowable temperature for the food drying process. For example, by using as a radiation shield 21 covering layer material with little sensitivity to temperature such as, for example, nitrogen free rice starch, the adverse effects of temperature radiation to the surface of the food material can be avoided. Such radiation in excessive amounts is undesirable because the photochemical decomposition frequently caused thereby can materially decrease the quality of the food product. Also, the amount of allowable temperature radiation frequently determines the maximum temperatures which can be applied to many high quality freeze-dried products. Theretore, the use of a radiation shield covering will permit the establishment of higher overall temperatures on the surface of the core :food substance. In this way the drying time for the food product can be substantially reduced.

In specific cases it is even possible to provide a higher temperature which will produce a desirable chemical change in the edible covering material. For example, the browning of a flour covering such as is frequently done intentionally in the preparation of food dishes could be accomplished during the freeze-drying process.

The superimposition of a covering layer need not necessarily cover the entire material being freeze-dried in all applications. It may be desirable to put the covering layer only upon part of the surface such as, for example, in the form of cross pieces which may be also used as spacers between the freeze-drying base product and the heated drying vessels.

As an extension of the present invention the outer covering layer can be adapted for use as a long term storage package for the base food product. One way of accomplishing this is to surround the base material with a porous layer of finely distributed thermoplastic or other substance having a sinter capacity. Then upon completion of the freeze-drying process and While the product is still under vacuum, the pores in the covering layer can be sealed by a brief application of heat. In this case, it proves expedient to include in the covering layer a material which is a good conductor of electricity so as to allow the use of inductive high frequency heating to accomplish sintering of the covering material.

Another method of providing such a storage cover for the food product is by the impregnation of a porous covering layer upon completion of the freeze-drying process.

Still another method of incasing freeze-dried material is to include in the covering layer one component of a temperable casting resin. Then after completion of the freeze-drying process, this layer can be impregnated with a second hardening resin component.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A method for treating water containing radioactive waste material which comprises the steps of packaging the water containing radioactive waste material with a filter material which is impermeable to solid radioactive material but porous to sublimating water vapors, and freeze-drying said packaged radioactive waste material by passing sublimating water vapors through said filter material so as to reduce the volume of radioactive waste material within said filter material package.

2. A method of freeze-drying which comprises the steps of preparing a substance for drying, coating said substance with a material which is porous to sublimating water vapor, freezing the substance to be dried, subjecting the frozen substance to a reduced pressure so as to cause sublimation of substantially all the frozen moisture in the frozen substance, and treating said coating material while said substance is maintained under vacuum so as to render said coating material impermeable thereby providing a vacuum tight package for said freeze-dried substance.

3. A method of freeze-drying according to claim 2 wherein said coating material has a sinter capacity and said treating step includes the application of sufiicient heat to said coating material to cause sintering thereof.

4. A method of freeze-drying according to claim 3 wherein said coating material includes a component having electrical conductivity and said treating step includes the inductive heating of said coating material.

5. A method of freeze-drying according to claim 2 wherein said treating step includes the step of impregnating said coating material.

'6. A method of freeze-drying according to claim 2 wherein said coating material includes a temperable casting resin component and said treating step includes the step of impregnating said coating material with a hardening resin component.

7. The method according to claim 1 wherein the radioactive waste packaging material is a filtration paper.

References Cited by the Examiner UNITED STATES PATENTS 2,577,704 12/1951 Flosdorf 99-199 2,713,003 7/1955 Rivoche 99204 2,785,075 3/1957 Malecki 99166 X 2,882,609 4/ 1959 Templeton 99204 X i r A. LOUIS MONACELL, Primary Examiner.

L. M. SHAPIRO, Assistant Examiner. 

1. A METHOD FOR TREATING WATER CONTAINING RADIOACTIVE WASTE MATERIAL WHICH COMPPRISES THE STEPS OF PACKAGING THE WATER CONTAINING RADIOACTIVE WASTE MATERIAL WITH A FILTER MATERIAL WHICH IS IMPERMEABLE TO SOLID RADIOACTIVE MATERIAL BUT POROUS TO SUBLIMATING WATER VAPORS, AND FREEZE-DRYING SAID PACKAGED RADIOACTIVE WASTE MATERIAL BY PASSING SUBLIMATING WATER VAPORS THROUGH SAID FILTER MATERIAL SO AS TO REDUCE THE VALUME OF RADIOACTIVE WASTE MATERIAL WITHIN SAID FILTER MATERIAL PACKAGE.
 2. A METHOD OF FREEZ-DRYING WHICH COMPRISES THE STEPS OF PREPARING A SUBSTANCE FOR DRYING, COATING SAID SUBSTANCE WITH A MATERIAL WHICH IS POROUS TO SUBLIMATING WATER VAPOR, FREEZING THE SUBSTANCE TO BE DRIED, SUBJECTING THE FROZEN SUBSTANCE TO A REDUCE PRESSURE SO AS TO CAUSE SUBLIMATION OF SUBSTANTIALLY ALL THE FROZEN MOISTURE IN THE FROZEN SUBSTANCE, AND TREATING SAID COATING MATERIAL WHILE SAID SUBSTANCE IS MAINTAINED UNDER VACUUM SO AS TO RENDER SAID COATING MATERIAL IMPERMEABLE THEREBY PROVIDING A VACUUM TIGHT PACKAGE FOR SAID FREEZE-DRIED SUBSTANCE. 